The apparatuses in question are in particular optical systems directing electromagnetic radiation, generated e.g. by lasers or LEDs as radiation sources, onto or into a material to be processed, and forming and focusing said electromagnetic radiation. Material processing can here e.g. be a patterning of material in the micro-range of the type executed e.g. in semiconductors or also in metallic materials. The present invention can especially be used for ophthalmologic optical systems, in particular in refractive cornea surgery, such as LASIK.
When material is processed by means of focused electromagnetic radiation, it is normally of decisive importance to precisely position the focus, in particular in the direction of the electromagnetic radiation (normally referred to as “z-direction”). The position of the focus is normally referred to as “focal position”. This term covers not only the above-explained location of the focus in the direction of the radiation (the so-called focus depth), but, more generally, also the position and the orientation of the focused radiation, i.e., by way of example, a displacement of the radiation with respect to the optical axis of the system or an angular position relative thereto.
US 2002/0171028 describes an apparatus for focus control, in which reflected light is made to interfere with a second beam through an optical imaging path, and an interferometric measurement and control are executed.
Also in U.S. Pat. No. 6,666,857 B2 focus control is executed by means of an interferometric wavefront control. Active wavefront control during photoablation on the human eye is accomplished by a combination of adaptive mirrors.
In US 2004/0021851 an optical array consisting of a laser and a subsequent beam shaping optics is used for measuring the focal length of an unknown lens. Measurement of the focal length is executed by focusing on a reference surface at different distances. The back-reflected part of the radiation is detected. The spot diameters are then evaluated with the respective distances. The focal length is determined by means of the “Newton” relation Z Z′=f2. A diffraction grating, which is not described in detail, is used for outcoupling the back-reflected part of the radiation. Also the Jones matrix formalism is used for calculating the focal length. The method has a precision of 1%.
WO 2007/096136 A1 describes an apparatus for detecting the focal position of an optical system with a partially reflective surface on the focus to be measured, a camera for recording an image that is reflected by said surface, and a computer for evaluating the image recorded by the camera. An optical element is arranged in the optical path of the optical system before the focusing imaging system, said optical element influencing said image in accordance with the focal position. The focal position is controlled through elements of the focusing optics.